1. Field of Invention
The present invention relates generally to the field of oilfield exploration, production, and testing, and more specifically to a material and soluble oilfield elements such as well operating elements and methods of using same, which exhibit a combination of rapid solubility and high strength and/or high toughness. This invention also relates to and altering the solubility of above mentioned high strength soluble oilfield elements and well operating elements.
2. Related Art
A diverter ball is a ball that is dropped or pumped through wellbore tubulars in a process known as diversion, and used during acidizing and fracturing operations. One common method of fracturing wells is to have a multitude of perforations open in the well that are exposed to the fracturing pressure being pumped into the well bore. The formation will begin to fracture behind a few of the perforations and the majority of the fracturing fluid will flow through these few perforations. Typically, initiating fractures requires more pressure than continuing an existing fracture so diverter balls are used to divert flow to other perforations. A percentage of diverter balls compared to the total number of perforations are dropped in a well (as an example, 10 balls are dropped at a time in a well with 100 perforations) and the balls theoretically will plug off the perforations which are taking the majority of flow because the flow is carrying the balls. Once the diverter balls are plugging the flowing perforations, the well bore pressure will increase until new fractures are started behind other perforations. More diverter balls are dropped until the majority of all of the perforated intervals are fractured. In other well operations, balls are used that function to open and close downhole valves in different fracturing zones, and may serve as temporary plugs to zones below the ball. In this sense the term balls includes bars, plugs, darts, and other shaped members, and are more generally referred to herein as well operating elements.
Diverter balls and fracturing elements are typically flowed to the surface or dropped to the bottom of the wellbore when their use is completed. If they are not degradable in the wellbore environment, there is a disincentive to their use. Dissolvable diverter balls are known in diversion operations, however, during use their strength and durability is not always considered. In a diversion process, a ball, dart, or other non-dissolvable fracturing element may land on a seat and be positioned effectively to divert fracturing fluid, acids, and/or proppants outwardly through crossover ports for flow through the annulus into the formation adjacent a perforated casing section. However, if these non-dissolvable fracturing elements are composed of non-engineered materials, they may dissolve and/or degrade in a shorter time period than desired or anticipated by the well operating personnel. Multiple small dissolvable diverter balls are then flowed in to the tubing, typically with the fracturing fluid, and seat on perforations in the casing to divert fracturing fluids, acids, and proppants from a high flow area of perforations to a low flow area so that the low flow area receives the treatment fluid. While eventually the dissolvable diverter ball material degrades, from mechanical action, contact with a fluid, heat, or combination thereof, it would be desirable if before the dissolvable material degrades that the materials remain tough and strong to perform their function, and not degrade prematurely.
It would be desirable in many well operations to have oilfield elements that are durable when required to be, but that also dissolve (or include a portion that dissolves) in an aqueous wellbore environment, without having to resort to highly acid conditions, high temperatures, and mechanical milling.
None of the known drop balls, diverter balls, valve elements and other known oilfield elements and tools have both the ability to perform functions requiring high strength and durability during their intended use, but also have the ability to dissolve when desired in aqueous environments, and there is a need in the art for such items.
The effect of addition of hydrophilic groups into polymers has been reported before, for example in Journal of Applied Polymer Science, Vol. 99, 756-774 (2006), Wiley Periodicals; and Journal of Materials Science, Vol. 40, 629-636 (2005); as well as in the following published patent documents: WO2005090742; US20050205266; US 20050205265; US20050205264; US20050173126. Published U.S. patent application Ser. No. 20050205265 lists the following polymers for downhole applications: degradable polymers selected from the group consisting of polysaccharides, chitins, chitosans, proteins, aliphatic polyesters, poly(lactides), poly(glycolides), poly(.epsilon.-carprolactones), poly(hydroxybutyrates), poly(anhydrides), aliphatic polycarbonates, poly(orthoesters), poly(amino acids), poly(ethylene oxides), polyphoshazenes, and mixtures thereof. The main drawback of these materials listed is that they do not dissolve rapidly enough. For example, polylactic acid (PLA) grade 3051D does exhibit very good tensile strength but shows almost negligible degradation/dissolution within 48 hrs at 220° F. (104° C.) in 4% sodium chloride brine.